1. Field of the Invention
The present invention relates to a semiconductor device provided with a MOSFET, and more particularly to a semiconductor device having an impurity region in a contacting portion of source and drain regions of the MOSFET and a metallic wiring and a manufacturing method thereof.
2. Description of the Prior Art
FIG. 23 shows a sectional structure of the conventional MOSFET. The withstand voltage of a MOSFET depends on the impurity distribution at the end of a drain on the side of a channel. For this reason, the high withstand voltage of the conventional transistor has been realized by optimizing the condition of forming source and drain diffused layers 6a, 6b and a channel region so as to relax the drain electric field.
Miniaturizing the device, however, has been accompanied by an increase in the impurity concentration within the device and a reduction in the depth of the source and drain diffused layers 6a, 6b. This strengthens the drain electric field and hence reduces the withstand voltage at the drain end.
On the other hand, at contact areas 10, in order to prevent metallic wirings 9 made of e.g. aluminum (Al) from sinking deeper than the source and drain diffused layers 6a, 6b to be brought into contact with the p-type substrate, an n-type diffused layers 11 is formed.
Since the diffused layer 11 can not be micro-miniaturized, at a certain point of development of micro-miniaturizing, the diffused layer 11 at the contact area sinks deeper than the source and drain regions 6a, 6b.
In this case, since the diffused layer 11 at the contact area has a higher impurity concentration than the low impurity diffused layer 6a of the drain, the withstand voltage at the contact area is lower than that of the drain diffused layer. Accordingly, the withstand voltage of the device depends on the contact diffused layer. Thus, realization of the high withstand voltage of the transistor requires that of the contact area.